[time-nuts] Analog Phase comparator

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Sep 21 17:47:27 EDT 2008


The AD9901KP is available in a side brazed DIP package (at least its 
listed as a current production device).
However Digikey for example, only stock the PLCC version.
With 10MHz inputs it should resolve a few tens of picosec of phase 
difference.
With a 10 bit ADC the resolution would be about 200ps.

If you need more resolution use a higher resolution ADC.
If you need less resolution either use a lower resolution ADC or use 
prescalers to reduce the AD9901input frequency.

To compensate for drift in the AD9901 gain one can
1) Use an ADC with its reference proportional to the AD9901 output stage 
collector current

or

2) Servo the AD9901 output stage collector current to the ADC reference

or

3) Measure the AD9901 output stage collector current periodically and 
use the measurement to correct phase detector output measurements for 
gain drift.


Thus an AD9901 can be used to meet both low end and higher resolution 
requirements.
Biased in ECL mode it has input differential amplifiers suitable for 
signals down to around a couple of hundred mV pp.

Since the AD9901 has a triangular phase detection characteristic a pair 
of them with one input signal inverted for the the second are required 
to determine the sign of the phase difference.
This is readily accomplished by swapping over the connections to one of 
the differential input buffers of the second AD9901.

Bruce



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